Stitich,
I'm in Australia and I'm using potting soil from a well known company over here called Amgrow. It does have bark in to help with drainage. The crunched up leaves you see in the pic are dead leaves of that particular plant.
I tested the soil ph and it's at 6.5-7. Which is a little high. The ferts should bring that down to about 6 shouldn't it?. I havn't tested the ph of the water, I'd assume it's ph nutral because I'm on tank water here. So the water is ala natural.
The Nutes I'm using is called "Thrive". It has 27-5-9 NPK. I started out feeding half strength. How certain are you it N deficiency?. I can just keep feeding it and it will come back. But if it's nute burn it's only gonna make it worse?.
There are three large plants showing similar signs while the other 15 are smaller and there leaves are slowly turning pale green. I hope your right. I hope it's not nute burn because I'm going to feen them again tonight.
Sometime next week they will be repotted into a coco/vermiculaite mix. So the bark will be gone
6.5 to 7 is fine you dont need to adjust it
and the nutrients your using im not familar with but it sounds like its a chemical fert which is way out of balance its got to high of nitrogen
that will be good now to fix your problem, yes you do have a nitrogen deficiency, trust me its not nute burn.
you need a balanced nutrients the one your using if you use it weekly like a normal fert your plant would build up to much nitrogen and have little phos potassium even though they are in veg right now
you need something like 10 5 5 or 15.10 10 something like that
anything over a npk ratio of 20 is a waste dont bother getting it
for bloom you need less nitrogen and more P K
newer leaves tend to turn pale green and darken up when they get bigger
as long as its nwer leaves and newer growths are pale green thats fine
but if they get bigger and are still pale then you may have a micro nute deficiency but right now you dont have any
you also need to make sure the nutrient you choose has micronutrients, it will list them in % on the back of the box
and remember never to listen to the back of the box it will burn your plants
you dont want to use any more than 1/2 teaspoon per gallon of water that goes for chemical fert, the one your using now is a chemical fert
your plants are far from a lost, they are fine they just need some food